I. Field of the Invention
This invention relates generally to an arrangement for selectively altering the input impedance of an operational amplifier, and more specifically to the adaptation of this approach to the operational amplifier employed in a demand-type pacemaker such that the system functions in a more reliable manner.
II. Discussion of the Prior Art
In the now abandoned application of David J. Fischer, Ser. No. 880,895, filed Feb. 24, 1978 and assigned to the assignee of the instant application, there is described a digital, programmable demand-type cardiac pacer system in which a pulse generator is adapted to be coupled by way of a catheter lead arrangement to the heart of the patient. As with all demand-type units, in the absence of naturally occurring heartbeat activity, the system functions to generate pulses at a desired rate for application to the heart of the patient. The same electrodes which are utilized to apply artificial stimulating pulses to the heart in the absence of naturally occurring heart activity are also used to pick up the electrical signals generated upon depolarization of the heart muscle and apply these signals to the R-wave sensing amplifier, the output of which is used to inhibit or reset the pulse generator so that it does not produce artificial stimulating pulses when normal R-waves are being produced on a regular basis. It is to be further noted that the system of the Fischer application includes a relatively large capacitor (the voltage doubling capacitor) in circuit with the pacer leads and, therefore, with the input of the R-wave sensing amplifier.
To achieve a desired gain from the operational amplifier, it is imperative that the input impedance of the amplifier be relatively high. In fact, the American Association for Medical Instrumentation (AAMI) has an established standard requiring an input impedance for a sensing amplifier used in the amplification of signals occurring upon the depolarization of body cells of at least 20 thousand ohms. It is found that the time constant of the input circuit to the R-wave sensing amplifier is quite large in that it is the product of the input impedance of the amplifier and the capacitance of the voltage doubling capacitor which determine that time constant.
When an artificial stimulating pulse is produced by the pulse generator and applied to the heart by way of the heart contacting electrodes, this large time constant may prevent sufficiently rapid recovery of the over-shoot appearing at the trailing edge of the applied impulse. If the time interval required for the over-shoot voltage to return to the normal base level is too long, it may exceed the refractory period of the system. As a result, the potential difference appearing across the input of the sensing amplifier at the completion of the refractory period may be sufficiently large to be interpreted by the system as an R-wave. When this happens, the system is deceived into believing that the heart is producing R-waves, thereby inhibiting, temporarily, the generation of artificial stimulating impulses. Thus, a patient suffering complete heart block would not be receiving the artificial stimulating impulses required for proper pacing.